I have no issue with people disagreeing with me.

I have no issue with people expressing an opinion that runs counter to one I believe firmly.

I do make an exception to this for opinions that are genuinely harmful - for example, I don't care how respectfully you go about phrasing it, I am going to lose every ounce of respect I can place in you if you try to convince me that vaccines are bad, and I don't care what they're like/interested in/etc., it's never okay to bully someone - but for most subjects I'm willing to hear someone I disagree with out, even if I know from the get-go I will disagree.

But please, for the love of all that is good and decent in this world, at least be smart about how you phrase your argument. Don't use ad hominem. Don't refuse to acknowledge a completely reasonable and rational statement by your opponent. Don't create this complete "us vs. them" mentality when the entire point of intelligent conversation is a mutual understanding, emphasis on mutual. Don't talk down and try to belittle your opponent, even if they're resorting to such grade school tactics themselves. Don't go into the argument looking specifically to make a scene.

Barring genuinely harmful beliefs, I have no problem with opinions being expressed even if I think they're completely stupid and dumb. Just don't be stupid about how you express them. Please. Few things make me more furious than a clearly intelligent person with something to say that says it in a dimwitted fashion and ruins their entire argument because of it.